Add the tomatoes, broth, wine, bay leaf, and parsley sprigs. Season with salt and black pepper. Bring to a boil, reduce the heat, and simmer 20 minutes. (The soup can be prepared in advance up to this point).

Add the fish, shellfish, and potatoes. Simmer over low heat until the fish has changed from translucent to opaque and the shellfish has opened, 10 or 15 minutes.

In a small bowl, whisk the cream with the egg yolk, and gradually ladle some of the hot soup into the egg mixture before adding the egg mixture to the soup. This step prevents the egg and cream from curdling. For the same reason, don't let the soup boil.